In optical communications networks, optical communications modules are used to transmit and/or receive optical signals over optical fibers. Optical receiver modules are optical communications modules that receive optical signals, but do not transmit optical signals. Optical transmitter modules are optical communications modules that transmit optical signals, but do not receive optical signals. Optical transceiver modules are optical communication modules that transmit and receive optical signals.
An optical transmitter or transceiver module has a light source that is driven by a driver circuit to cause the light source to generate amplitude and/or phase and/or polarization modulated optical signals that represent data. The modulated optical signals are optically coupled onto an end of an optical fiber by an optics system of the module. The light source is typically a laser diode or light emitting diode (LED). The optics system typically includes one or more reflective (e.g., mirrors), refractive (e.g., lenses) and/or diffractive (e.g., gratings) elements.
An optical receiver or transceiver module includes a photodetector (e.g., a p-doped-intrinsic-n-doped (PIN) diode) that detects an optical data signal passing out of an end of an optical fiber and converts the optical data signal into an electrical signal, which is then amplified and processed by electrical circuitry of the module to recover the data. An optics system of the module optically couples the optical data signals passing out of the end of the optical fiber onto the photodetector.
As the demand for data throughput continues to increase, the data rate, or bandwidth, of optical links is being pushed ever higher. While various transceiver and optical fiber link designs enable the bandwidth of optical fiber links to be increased, there are limitations on the extent to which currently available technologies can increase the bandwidth of an optical link. One way to increase the bandwidth of an optical link is to use multi-channel optical communications modules, also known as parallel optical communications modules, which transmit and receive optical data signals over multiple channels in parallel. Another way to increase the bandwidth of an optical link is to use wavelength division multiplexing and demultiplexing (WDM) to enable multiple optical data signals of different wavelengths to be sent over the same optical fiber. Yet another way to increase the bandwidth of an optical link is to transmit and receive optical signals of different wavelengths over each optical fiber, which is commonly referred to as bidirectional (BiDi) communications.
While multi-channel optical communications modules that incorporate WDM and/or BiDi functionality exist, they are very challenging to design and manufacture. One challenge is achieving sufficiently precise optical alignment between the light sources and the optical elements of the optical coupling system and between the photodetectors and the optical elements of the optical coupling system. If sufficiently precise optical alignment is not achieved, performance will be degraded. To date, optical communications modules that incorporate WDM use single mode optical fibers (SMFs) in the optical link, which have a diameter of around 10 micrometers (microns). With fibers that are this small in diameter, a variety of passive and active alignment devices and techniques are needed to achieve sufficiently precise optical alignment. Active alignment devices and techniques can be expensive and time consuming to perform.
Another challenge with designing and manufacturing multi-channel optical communications modules that incorporate WDM and/or BiDi functionality is sufficiently reducing back reflection of light from the end face of the optical fiber into the aperture of the light source. If back reflection is not properly managed, performance will be degraded. Therefore, the optical coupling system needs to be designed and manufactured to sufficiently reduce the amount of back-reflected light that is incident on the aperture of the light source.
Another challenge with designing and manufacturing multi-channel optical communications modules that incorporate WDM and/or BiDi functionality is matching the light modes of the light produced by the light source with the light modes of the optical fiber. The launch conditions provided by the optical coupling system when launching light into the end face of the optical fiber need to be such that the light modes of the light produced by the light source closely match the light modes of the optical fiber. If such mode matching is not achieved, the optical link will experience high relative intensity noise (RIN) that will degrade performance. Mode matching is more of a problem in cases where multimode optical fiber (MMF) is used due to the fact that MMFs exhibit greater mode dispersion than SMFs. For that reason, MMFs are typically only used in relatively short optical links. MMF has a much larger diameter (e.g., 60 microns) than SMF, and therefore optical alignment is less difficult to achieve. MMF is also less expensive than SMF, but as indicated above, exhibits higher mode dispersion than SMF, which leads to higher RIN.
A need exists for an optical communications module that has WDM capabilities for increased bandwidth, that is suitable for use with MMF, that uses passive alignment devices and techniques, that can be manufactured cost effectively, and that can be relatively easily assembled.